fix: de-flake celery retry test

task_success fires async; wait for the update count before asserting.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: snopoke

tests/test_celery.py

@@ -316,6 +316,7 @@ def add_retry(self, a, b, is_retry=False):
         get_task.return_value = task_for_test()
         result = add_retry.delay(2, 2)
         assert result.get(timeout=10, propagate=True) == 4
+        _wait_for_call_count(update, 4)
 
     assert create.call_count == 2
     assert update.call_args_list == [
@@ -422,6 +423,7 @@ def task_signature(self, a):
 
         result = chain.delay()
         assert result.get(timeout=10, propagate=True) == 16
+        _wait_for_call_count(update, 6)
 
     assert create.call_count == 3
     assert get_task.call_count == 3